The present invention relates, in general, to optical systems and, more particularly, to an integrated optical receiver.
The use of optical interconnect systems is gaining wide spread acceptance for transferring data at high speeds. Applications such as Local Area Networks (LANs), Multimedia, and the Internet are integrating optical solutions into their systems because they have the capability of meeting present and future needs. The main competition to an optical interconnect system is a copper interconnect system. Copper interconnect systems are inefficient at transmitting data at transfer rates exceeding 300 megabits/second. Copper interconnect systems are also lossy, making long runs of interconnect impossible without intermediate amplification stages. Unlike copper interconnect, optical (or fiber) interconnect has extremely high bandwidth and low loss. In particular, optical interconnect is ideal for data transfer in the 300 megabit/second to 40 gigabit/second range for the applications described hereinbefore.
FIG. 1 is a schematic diagram of a prior art transmitter system 11 and a receiver system 12 for sending data across an optical fiber 13. Transmitter system 11 converts a plurality of electronic signals into a stream of data bits. Transmitter system 11 comprises a multiplexer (MUX) 14, a laser driver 15, and a laser diode 16. The high speed of data transmission of an optical system allows multiple data channels to be ported through a single link. Providing more than one data channel ensures that the optical link is highly utilized. Multiplexer 14 has a plurality of inputs and a single output. Multiplexer 14 is a switching circuit coupling one of the inputs to the output. Laser driver 15 receives a signal from multiplexer 14 and has an output coupled to laser diode 16. Laser driver 15 enables laser diode 16 to transmit an optical signal corresponding to the electrical signal from multiplexer 14. Laser diode 16 is connected to optical fiber 13 for carrying the optical signal.
Receiver system 12 comprises a photodiode 17, a preamplifier 18, an amplifier 19, a clock extraction circuit 20, a frequency divider circuit 21, a decision circuit 22, and a demultiplexer 23. Photodiode 17 is a reverse biased diode sensitive to the frequency of light emitted by laser diode 16. Photodiode 17 is connected to optical fiber 13. Light transmitted through optical fiber 13 is absorbed by photodiode 17 creating electron-hole pairs. An electric field across reverse biased photodiode 17 forms a current with the electron-hole pairs corresponding to the light being transmitted.
Preamplifier 18 amplifies the small current signal generated by photodiode 17. The small current signal can be either an analog or a digital signal. In general, preamplifier 18 is a transimpedance amplifier that converts the optically generated current to a voltage signal. Amplifier 19 further amplifies the voltage signal produced by preamplifier 18. The amplified signal of amplifier 19 requires further processing or wave shaping to conform to a signal type being used at the receiving end. In general, high speed digital data transmission does not produce signals with sharply defined transitions. For example, a high speed digital transmission may look more like a sine wave than a square wave. Decision circuit 22 performs the wave shaping of the amplified signal from amplifier 19. Data transmission error is minimized by having decision circuit 22 sense the amplified signal at a maxima or minima, providing an increased signal to noise ratio. Clock extraction circuit 20 generates a clock signal for decision circuit 22 that is centered about the maxima and minima of the small signal current signal of photodiode 17. Decision circuit 22 outputs a signal to demultiplexer 23 corresponding to the small current signal generated by photodiode 17. Frequency divider circuit 21 generates a signal from the clock signal of clock extraction circuit 22 to select a data channel output of demultiplexer 23. The signal provided by decision circuit 22 is output at the selected data channel output of demultiplexer 23.
Integrated optical receiver circuits being offered in the marketplace that operate at transmission rates greater than one gigabit/second are typically implemented in gallium arsenide (GaAs). The GaAs integrated optical receiver circuits comprise a photodetector and a transimpedance amplifier. High levels of integration are not easily accomplished in GaAs nor is it cost effective. In general, a two chip solution is required to build a complete optical receiver system.
Accordingly, it would be advantageous to have an integrated circuit capable of high levels of integration with an efficient photodetector and a method for manufacturing the integrated circuit. It would be of further advantage for the photodetector to operate at frequencies up to 3 gigabits/second with a substantially reduced cost.